The term “impact compactor” or “impact roller” typically refers to a soil compaction device which includes one/two compactor drums of non-round shape which, when towed/driven over a soil surface, produces a series of periodic impact blows on the soil surface (see FIG. 1). These periodic blows compact the soil which results in packing and orientating the soil into a more dense and effective particle arrangement, which reduces air voids and prevents further densification and shear failure of the soil. The compactor drums of the impact compactor each has a series of spaced apart salient points on its periphery with each such salient point followed by a compacting face. As the impact compactor is towed over the soil surface, for instance by means of a tractor, the compactor drum rises up on each salient point and then falls forwardly and downwardly as it passes over that point, with the result that the following compacting face applies an impact blow to the soil surface (see FIG. 2). The function of the compactor drum is therefore to store potential energy as it rises up on each salient point and then to deliver this energy as an impact blow.
In order to achieve the required degree of compaction, a predetermined number of passes is normally applied by the impact compactor to the site. After the predetermined number of passes has been carried out, soil tests are conducted at isolated discrete positions in order to establish whether the required degree of compaction has been achieved. Although these soil tests are only conducted on a very small ratio of the total area undergoing compaction (usually no more than one in one hundred thousandths of the area being compacted) the test results are extrapolated to indicate whether the soil has reached the required degree of compaction; still requires further passes of the impact compactor; or has already exceeded the required degree of compaction. It is therefore often incorrectly assumed that the site has been adequately compacted when in fact portions of the site remain inadequately compacted. Poorly compacted soil can result in costly premature failure of whatever road, railway line, airport runway or other structure the soil may in future be required to carry.
Impact compactors have proved to work well in achieving high levels of soil compaction, even at substantial depths below the soil surface. This allows for the achievement of greatly improved uniformity of soil strength over a site, provided that the entire site is rolled to refusal or near refusal of settlement. However, it is difficult to determine when refusal of settlement has been reached over an entire work site as some areas may reach refusal of settlement earlier than others, resulting in insufficient or superfluous compaction over large areas of the work site. Different soil types may have different elastic properties once refusal of settlement has been achieved and it is therefore important to measure these elastic properties to ensure uniformity of the achieved soil strength.
It can be shown that the amount by which the drum of the impact compactor penetrates into the soil during an impact blow is directly related to the soil strength. Large penetration measurements would correlate to low soil strength and small penetration measurements would correlate to high soil strength. Once the soil refuses further settlement, the penetration measurements achieved will remain constant, indicating that any deformation of the soil achieved is elastic. Elastic deformation occurs when there is a temporary change in the shape of the soil which is fully recovered when the applied stress (the compactor drum) is removed. The response of the soil to unload is immediate. Plastic deformation occurs when there is a permanent change in the shape of the soil which is not recovered when the applied stress is removed. When there is no more plastic deformation during an impact blow, it means that the impact compactor has reached the limit of its compaction capability and cannot improve the soil strength any further. It can be said that the soil reaction force has reached a form of equilibrium with the pressure applied by the falling compactor drums.
The effect which the compactor drums (due to their shape) have on a ground surface over which they travel can be visually illustrated as shown in FIG. 3, with the ground forming a sinusoidal pattern. The upper sinusoidal wave in FIG. 3 illustrates the pattern formed on the first pass where the depth of penetration may be as great as 150 mm or more. The middle and lower waves illustrate how the amplitude of the sinusoidal wave reduces as the impact compactor completes more passes and the ground gets harder. The impact compactor, however, does not deliver an impact blow to the same spot every time and the sinusoidal patterns of each pass will therefore overlap.
Soil density is extensively used by the construction industry to specify, estimate, measure, and control soil compaction even though it is not usually the most relevant engineering property for determining whether the ground is well compacted. This practice was adopted long ago because soil density could be easily determined from measurements using devices such as a nuclear density gauge which is commonly used today.
Current methods for measuring soil strength are relatively slow, labour-intensive and/or lack accuracy. Construction sites are often under sampled, causing inadequate compaction to go undetected or feedback to be provided too late for the cost-effective correction of any problems.
It is an aim of the present invention to provide means which will at least alleviate some of the above-identified problems.